In underwater vehicles, especially those that are unmanned, it is desirable to provide a self-contained power source to propel the vehicle. Some underwater vehicles, such as torpedoes, employ dry cell batteries to provide the necessary power. However, these batteries have a limited shelf life and if the vehicle is not used within a predetermined period of time, the vehicle is either scrapped or the batteries must be replaced.
It is also known to provide underwater vehicles with fuel cells which have an unlimited shelf life. Fuel cells are simple static devices that convert chemical energy in a fuel directly, isothermally, and continuously into electrical energy. Fuel and oxidant (typically oxygen or air) are fed to a fuel cell stack in which an electrochemical reaction takes place that generates electrical energy for powering the underwater vehicle. A coolant may also be supplied to the fuel cell stack to dissipate excess heat and maintain efficient operation of the fuel cell. Some fuel cells employ a moving anode configuration, wherein as the anode material is consumed, the anode is moved to maintain a predetermined distance between the anode and the cathode.
One important aspect of the operation of fuel cells with a moving anode is that the oxidizing agent, the electrolyte and the coolant must be maintained at substantially equivalent pressures within the fuel cell. Otherwise, unequalized pressure may cause structural failure of the cathode within the fuel cell and/or the entire cell structure. Accordingly, failure of the fuel cell causes failure of the underwater vehicle. One approach to overcoming the aforementioned problem has been to provide the fuel cell stack with active controls of the inlet pressure of the coolant and the electrolyte to match the inlet pressure of the oxidizing agent. It will be appreciated that such an active control system requires a plurality of sensors with sophisticated pump and valve control systems to maintain the desired pressure equalization. It will be appreciated that these pumps and valves add cost to the underwater vehicle and are prone to failure in undersea conditions. Additionally, these additional components increase the weight of the underwater vehicle and reduce its performance.
Based upon the foregoing, it is evident that there is a need in the art for a simplified pressure equalization scheme for fuel cell stacks used in underwater vehicles.